It is well-known that the modulated (transmitted) bandwidth of an FM signal is larger than its baseband input signal. The multiplex baseband signal of an FM stereo broadcast has a one-sided bandwidth of 53 kHz. However, its frequency modulated output extends over an effective bandwidth of roughly 260 kHz (±130 kHz). Although the FM signal has a theoretically infinite bandwidth, the power spectral density at ±130 kHz is typically 40 dB lower than at its center frequency. Hybrid In-Band On-Channel (IBOC) FM systems utilize the channel spectrum for digital subcarriers on either sideband of the host FM analog signal. The digital subcarriers can be located in frequency ranges from 129 kHz to 199 kHz on either side of the host FM analog signal. The basic hybrid system places subcarriers sufficiently separated in frequency from the analog FM such that the mutual analog and digital interference is minimal and acceptable for broadcast quality. Optional digital subcarriers can extend closer to the FM host to permit increased digital capacity for optional digital services, depending on the hybrid mode of operation. The optional digital subcarriers can extend as close as 122, 115, or 101 kHz, depending on the hybrid mode of operation. However the analog FM signal has significant power in these frequencies and affects the utility of this portion of the spectrum for digital subcarriers.
There is a need for a method and apparatus that modifies the baseband multiplex signal in such a manner that is compatible with constant-envelope high power amplifiers (HPAs) in IBOC signal transmitters, while limiting or eliminating any resulting additional distortion to the demodulated audio in a receiver.